1. Field of the Invention
The present invention relates to a method of joining metallic parts. In particular, the present invention relates to a method of joining superalloy sub-components and repairing a component by removing a damaged portion and re-inserting a replacement section.
2. Description of Related Art
Industrial gas turbine (IGT) blades are produced in one of three basic forms: equiaxed, directionally solidified or single crystal. In any of these cases, castings for IGT blades have become extremely large and costly. As the blades become larger and the casting processes become more sophisticated, casting yields tend to decrease, resulting in extra re-melting and casting costs in an effort to recycle defectively produced blades. Low casting yields of large turbine blades also make large volume manufacturing inefficient and expensive. This decrease in casting yields can be attributed to the fact that the casting of large sections inherently induces more defects. To avoid this issue, many manufacturers have been finding ways to join smaller blade components, known as sub-assemblies, to form a single large turbine blade.
Moreover, modern high temperature superalloy articles, such as nickel-based, precipitation strengthened superalloys used in the manufacture of rotating gas turbines blades, comprise complex alloys at the cutting edge of high temperature metallurgy. Over the years, superalloy materials have been developed to provide mechanical strength to turbine blades and vanes operating at high temperatures. As these turbine blades are difficult and expensive to manufacture, it is desirable to repair a damaged blade than to replace one.
Typically, aero and IGT hot section components are repaired using either welding or brazing methods. Both methods have been successfully applied to a variety of hot section turbine component materials including nickel-, cobalt-, and iron-based superalloys. Stationary components, such as vanes (also known as nozzles), transition pieces, or combustor liners or combustors are the most often repaired hot section components. Repairs on stationary components may be performed over virtually the entire component due to the lower stresses experienced during operation because these components experience only operational and thermal stresses and do not experience the high rotational stresses experienced by blades (buckets) or discs.
However, various issues or limitations appear when brazing or welding methods have been used to repair turbine blades. For example, narrow gap brazing techniques have been plagued by joint contamination that results in incomplete bonding, even when elaborate thermo-chemical cleaning processes precede the brazing operation. Narrow gap brazing also lacks the ability to restore damaged or missing areas on the blade. Joints formed using wide gap brazing methods can be difficult to set-up, and porosity in the deposited filler material continues to be a concern. Gas Tungsten Arc Welding (GTAW) and Plasma Transferred Arc Welding (PTAW), while the most commonly used methods for blade repairs today, require the use of lower strength fillers in order to avoid cracking, which limits which parts of the blade that can be welded.
Due to these limitations, blade repairs are limited to the lower stress regions of the blade airfoil. Thus, some 80 to 90 percent of blade surfaces are non-repairable, and such non-repairable blades are generally returned to suppliers as scrap for credit against replacement blades. The financial impact of this is significant for the utility industry. For example, a single air-cooled, row 1 rotating blade may cost up to thirty-five thousand dollars to replace, and, depending on the turbine manufacturer and model, there are approximately 90 to 120 blades in a typical row. Thus, the need to develop an improved method to repair damaged blades by joining the damaged blade with a replacement superalloy piece would be desirable.
As noted, it would be desirable to provide an improved technique for joining superalloy parts, such as superalloy turbine blade sub-assemblies, that are subject to high temperatures and stresses, including operational, thermal, and rotational stresses. It also would be desirable to provide an improved technique for repairing damaged turbine blades by joining the damaged blade with a replacement superalloy piece, regardless of the location of the damaged area.